Russia may possess more than half of the highly enriched uranium in the world and a new report says this poses “significant nuclear risks.”

A report published today by the International Panel on Fissile Materials (IPFM), an independent research group based at Princeton University, claims that Russia possesses the world's largest stockpile of highly enriched uranium—the main ingredient in nuclear weapons—and that this stockpile poses "significant nuclear risks" and hampers international efforts to reduce the production of enriched uranium.

The authors' most pressing concern is that highly enriched uranium (HEU) intended for civilian use could fall into the wrong hands as a result of theft, which has occurred dozens of times around the world since 1993.

"If you have large amounts of material and if that material is in active use, there is always a non-zero risk of loss of that material," Pavel Podvig, an IPFM nuclear researcher and lead author of the study, told me in an email. "Given that highly enriched uranium is a material that can be used to build a very simple or even improvised nuclear explosive device (unlike plutonium that requires more design sophistication) that risk cannot be ignored."

The IPFM is a panel consisting of nuclear experts from 16 countries, and has advocated for banning HEU production for civilian use in the past. Their new report estimates that Russia has a stockpile of 680 tons of highly enriched uranium, which may account for over half the total amount of HEU in the world. The US, by comparison, has around 600 tons of HEU.

While this flies in the face of international attempts to limit the amount of HEU in circulation, a number of countries benefit from Russia's surplus of uranium, including France and Germany, which use it to power domestic reactors.

According to Podvig, estimates of Russia's HEU stockpile were obtained by analyzing publicly available documents published by Rosatom—the Russian nuclear authority—about the country's nuclear activities.

At the moment, only 26 countries have stocks of highly enriched uranium, but Russia has the distinction of being the sole country producing it for export. According to Podvig, the HEU was produced at four "very large" enrichment plants built by the Soviet Union mostly to enrich uranium for nuclear weapons. As a result of the push for disarmament after the Cold War, Russia is now sitting on top of a lot of a lot of weapons-grade HEU that it doesn't need, and it continues to produce more non-weapons grade uranium for other countries.

In nature, uranium ore consists of 99.3 percent of uranium-238 isotopes and .7 percent uranium-235 isotopes on average. The difference between these two 'types' of uranium comes down to only three neutrons, but these three particles mean the difference between a nuclear bomb and a chunk of rock.

Only uranium-235 can sustain the fission reaction that makes a nuclear weapon do its thing, but the infinitesimal amounts found in nature won't quite cut it.

Anything above 20 percent enrichment of U-235 is considered highly enriched and can be used to create a weapon. In order to weaponize uranium in a bomb, it has to be enriched to about 90 percent uranium-235, although far more dilute uranium can be used for a relatively simple "gun-type nuclear weapon." The enriched uranium ore used to power nuclear reactors for civilian uses, on the other hand, can have wildly different requirements, ranging from just over 20 percent to nearly 90 percent.

As the report notes, Russia hasn't made the reduction of its highly enriched uranium a priority. In fact, it seems to be openly defying international efforts to reduce the amount of HEU in circulation by actively pursuing a number of new civilian projects that use it, as well as resuming production of HEU for export in 2012.

According to Podvig, this includes 90 percent enriched uranium for the FRM-II reactor in Germany, as well as 64.4 percent enriched uranium as fuel for China's Experimental Fast Reactor.

As noted in the new report, Russia is also making use of its HEU stockpiles at home. Highly enriched uranium is used for civilian purposes, particularly to produce neutrons in research reactors or isotopes for medical research. Russia currently operates 58 facilities that use HEU, which accounts for a little over half of the 115 HEU sites globally. In addition, the Russian navy has a fleet of 58 nuclear-powered ships that also use HEU.

"My understanding is that the material used for naval reactors or breeder reactors, which is less than 90 percent enriched, is produced, while the 90 percent HEU for domestic reactors is taken from the old stock," said Podvig.

As Podvig pointed out, there is no binding agreement that would require Russia or any other country to reduce their nuclear stockpile. Although 21 countries pledged to reduce their HEU stockpiles at the Nuclear Security Summit last year, Russia was not among the signatories to the pledge. Neither were France and Germany, both of which use Russian HEU for domestic reactors.

"Right now, the technical community in Russia does not seem to be convinced that HEU minimization is worth the trouble," Podvig said. "In a way, they have a point—if you focus only on civilian research reactors or medical isotope production, as the US does, then you are not dealing with other, more important aspects of the issue, such as the use of HEU in naval reactors, for example."

Although the US committed to reducing the civilian use of HEU during the Nuclear Security Summit, this ignores the use of HEU in ships, which accounts for over half of total HEU use globally. According to a 2014 report in the New York Times, naval reactors use four times more HEU annually than research reactors, and US ships account for roughly three-quarters of HEU use in naval reactors globally.

In other words, if the US was really committed to reducing non-weapon HEU use, it should look toward scaling back its naval reactors. Until then, Podvig said that creating a formal treaty for HEU reduction probably isn't the most effective way to address the issue.

"If the United States and others would make a strong case for reducing the use of HEU, Russia can be brought on board," Podvig said. "Technical communities in the US and Russia have always been able to work together very well. It would be good to encourage contacts between scientists because overall, engaging the technical community would be the most productive way forward."